This invention relates to semiconductor lasers and more particularly to single and multiple quantum well lasers that may have their operating or primary emission wavelength tuned to a selectively different emission wavelength for particular applications, e.g., optical storage systems or optical communication systems, or for periodic adjustment of the emission wavelength to a predetermined and desired value as in spectroscopic applications or in variable frequency communication systems.
Semiconductor lasers are used in a wide variety of applications requiring desired and fairly precise intensity, optical output power and wavelength of operation. In fabricating a semiconductor laser by available epitaxial processes, it is not possible to know precisely what the predominant emission wavelength will be in the wavelength gain spectrum of the laser.
One manner previously used in selecting an operating wavelength for a semiconductor laser is the provision of an external diffraction grating aligned in the path for optical emission from one facet of the laser. The grating provides, with appropriate lenses, the required feedback to support lasing conditions and creates a coupled cavity which becomes an operating segment of the Fabry-Perot cavity formed by the cleaved surfaces of the laser. The grating also provides selective wavelength feedback so that a single oscillation frequency with narrow linewidth may be selected and is selectable over the spectral emission of the semiconductor laser. Examples of the art related to wavelength selective, external grating cavity techniques are found in the following articles and referenced articles cited therein: J. A. Rossi et al., "Time Delays in External-Cavity-Controlled GaAs/Ga.sub.x Al.sub.1-x As Single-Heterostructure Diode Lasers", Applied Physics Letters, Vol. 23(5), pp. 254-256 (Sept. 1, 1973); J. A. Rossi et al., "High-power Narrow-Linewidth Operation of GaAs Diode Lasers", Applied Physics Letters, Vol. 23(1), pp. 25-27 (July 1, 1973); Thomas L. Paoli et al., "Single Longitudinal Mode Operation of CW Junction Lasers by Frequency-Selective Optical Feedback", Applied Physics Letters, Vol. 25(12), pp. 744-746 (Dec. 15, 1974); P. D. Wright et al., "Homogeneous or Inhomogeneous Line Broadening in a Semiconductor Laser: Observations on In.sub.1-x Ga.sub.x P.sub.1-z As.sub.z Double Heterojunctions in an External Grating Cavity", Applied Physics Letters, Vol. 29(1), pp. 18-20 (July 1, 1976); P. D. Wright et al., "In.sub.1-x Ga.sub.x P.sub.1-z As.sub.z Double Heterojunction Laser Operation (77.degree. K., Yellow) in an External Grating Cavity", Journal of Applied Physics, Vol. 47( 8), pp. 3580-3586 (August 1976); and R. Wyatt et al., "External Cavity Laser with 55 nm Tuning Range", Electronics Letters, Vol. 19(3), pp. 110-112 (Feb. 3, 1983).
In most of these prior art exemplifications of external grating tuning, a semiconductor laser, with one of its output facet having an antireflection (AR) coating, has its output emission focused on an external grating provided single mode oscillation with narrow linewidths over wavelength ranges, in the best of cases, as much as 550 .ANG., which range is within the optical emission spectrum of the laser. See the last mentioned reference in the paragraph above.
Dye lasers are known to have a relatively wide tuning wavelength range compared to p-n junction diode semiconductor lasers which are considerably more limited primarily due to the fundamental fact that excess carriers thermalize rapidly to a small energy range near each band edge. The width of the recombination radiation spectrum in a conventional heterostructure laser of GaAs/GaAlAs is determined mainly by the active region energy gap but with some influence due to the concentration and type of impurity doping employed in the active region. A Si-doped GaAs active region, for example has provided for comparatively one of the widest energy range of tunability for semiconductor lasers. The tunable energy range, for example, is about 30 meV to 40 meV for these conventional lasers.
We have discovered that the range of wavelength selection can be materially increased as well as the selectivity of operating wavelength within the tunable energy range materially increased by employing a semiconductor injection laser, having as an essential feature, a quantum well active region. For example, we have successfully tuned across energy ranges of 130 meV and more.